This invention relates to compression release engine brakes, and more particularly to apparatus for controlling, adjusting, or modifying the timing or other related characteristics of the operation of compression release engine brakes.
As shown, for example, in Cummins U.S. Pat. No. 3,220,392, a compression release engine brake or retarder may be mounted on an internal combustion engine to temporarily convert the engine from a power source to a power consuming gas compressor. An engine brake performs this function by using an appropriately timed mechanical input from one part of the engine to open an exhaust valve or valves in an engine cylinder which is nearing top dead center of its compression stroke. This allows the gas compressed in that cylinder to escape to the exhaust manifold of the engine, thereby preventing the engine from recovering the work of compression during the subsequent "power" stroke of the cylinder. (Of course, the fuel supply to the engine is typically turned off during operation of the engine brake.) In this way the engine brake helps to slow down or retard the engine and the vehicle propelled by the engine, thereby reducing the need to use the ordinary wheel brakes of the vehicle. This prolongs the life of the wheel brakes and increases vehicle safety.
In order to perform the function described above, a compression release engine brake typically includes hydraulic circuits for transferring the above-mentioned mechanical inputs to the exhaust valves to be opened. Each such hydraulic circuit has a master piston which is reciprocated in a master piston bore by the associated mechanical input from the engine. Hydraulic fluid in the circuit transmits the motion of the master piston to a slave piston in the circuit. Thus the slave piston reciprocates in a slave piston bore in response to the flow of hydraulic fluid in the circuit. The slave piston acts, either directly or through the exhaust valve opening mechanism of the engine, on the exhaust valve or valves to be opened, thereby opening the exhaust valve or valves at the appropriate times.
The timing of the exhaust valve openings described above is critical to the performance of the engine brake. Slight differences in timing can greatly affect the braking horsepower produced, as well as such other performance characteristics as the stress imposed on various components of the engine and engine brake. For example, delaying the initial opening of the exhaust valve until closer to top dead center of the compression stroke typically increases the engine braking available, but if the delay is too great, unacceptably large forces may be required to open the exhaust valves. As shown in Custer U.S. Pat. No. 4,398,510, hydraulic lash adjustors are known for controlling the gap between the slave piston and the associated exhaust valve mechanism for controlling this aspect of engine brake timing. While highly successful, these hydraulic lash adjustors may take several cycles of engine brake operation to become effective when the engine brake is turned on, and they may also take some time to deactivate after the engine brake is turned off. The initial delay in effectiveness may mean that full engine braking is not initially available, and the subsequent delay in deactivation may interfere with a few cycles of engine operation with fuel present in the engine cylinders. This latter operating characteristic can cause uncombusted fuel to be exhausted by the engine. This is both wasteful and environmentally undesirable.
Another respect in which it may be desirable to modify the timing or motion of the slave piston is to "clip" that motion as shown, for example, in Hu U.S. Pat. No. 5,201,290. This is typically accomplished by releasing some hydraulic fluid from the hydraulic circuit after a certain amount of motion of the slave piston has been produced. This may be desirable so that a strong hydraulic pulse from the master piston can be used to produce precisely timed exhaust valve opening, while the clipping action prevents excessive travel of the exhaust valve or undesirably prolonged opening of that valve. Excessive travel of the exhaust valve is to be avoided because it may result in contact between the exhaust valve and the associated piston. Prolonged opening of the exhaust valve may be undesirable because it may result in a back flow of gas from the exhaust manifold into the engine cylinder when the exhaust valve for another cylinder opens.
The known slave piston clipping mechanisms (e.g., as shown in the Hu patent mentioned above) are spring-loaded followers which travel with the slave piston until a follower stop is reached. Separation of the follower from the slave piston opens a passageway through which hydraulic fluid can escape from the circuit, thereby stopping the stroke of the slave piston.
Somewhat related slave piston "reset" mechanisms are shown in Cavanagh U.S. Pat. No. 4,399,787.
Again, clip valve and reset mechanisms such as those described above have been highly successful, but because they are passive they cannot perform all slave piston clipping or resetting functions that it would be desirable to perform in some cases. For example, it may be desirable to prolong exhaust valve openings at higher engine speeds. This would help to ensure that there is sufficient time for the compressed gas to escape from the engine cylinders, especially if the engine is turbocharged and the mass of gas in the cylinders is therefore higher at higher engine speeds when turbocharger effectiveness is greater. However, the known passive clip valve and reset mechanisms cannot produce different clipping or resetting effects at different engine speeds.
In view of the foregoing it is an object of this invention to provide improved compression release engine brakes.
It is another object of this invention to provide apparatus which can actively and substantially instantaneously affect the motion of the slave pistons in a compression release engine brake.